1. Field of the Invention
The present invention relates to an electrophotographic photoconductor containing a novel azo compound, a process for forming an image, an apparatus for forming an image and a process cartridge for the apparatus.
Specifically, the present invention relates to novel azo compounds and materials for producing the azo compounds, and the method for producing them, in particular, the azo compounds useful as an organic optical conductor, coupler compounds, which are the material for producing the azo compounds, and the method for producing them.
2. Description of the Related Art
Conventionally, there have been known optical conductive photoconductors used for electrophotography, which are largely divided into various kinds of inorganic optical conductors and organic optical conductors. “System for electrophotography” referred to herein is an image forming process, so-called Carlson Process. In the Carlson Process, generally, first, the optical conductive photoconductor is charged in the dark, by, for example, corona discharge treatment. Thereafter, an image is exposed to light to selectively scatter charges from exposed portions only in order to obtain a latent electrostatic image, and finally the electrostatic image is developed with toner made of a mixture of coloring agents such as pigment, dye, and the like, and a polymeric material to visualize images for formation. Since the photoconductor using the organic optical conductor has an advantage over those using the inorganic optical conductor within freedom, a photosensitive wavelength band, film forming ability, flexibility, transparency of a film, mass-productivity, toxicity, cost, and the like, at present, the organic optical conductor is used for most of the photoconductors. The photoconductors repeatedly used in the electrophotographic system and the similar processes are required to have electrostatic properties as typified by sensitivity, acceptable potential, potential retainability, potential stability, residual potential, spectral sensitivity characteristic, and the like.
From the above-mentioned viewpoint, there are known organic optical conductors, which have been proposed and practically used, such as azo compounds (disclosed in Japanese Patent Application Laid-Open (JP-A) No. 54-22834 and JP-A No. 61-151659), phthalocyanine compounds (disclosed in JP-A No. 48-34189 and JP-A No. 57-14874), perylene compounds (disclosed in JP-A No. 53-98825 and JP-A No. 63-266457), a polycyclic quinone compound (disclosed in JP-A No. 61-48861), squaleririum compounds (disclosed in JP-A No. 49-105536 and JP-A No. 58-21416), and the like.
The azo compounds, in particular, are easy to synthesize, have considerably free molecular design, and they widely vary in electrophotographic properties and spectral sensitive regions according to their differences in molecular structures of azo components, coupler components, and binding modes, and the like.
Therefore, a great deal of study on the azo compounds has been done not only on analog recording photoconductors but also on digital recording photoconductors.
This means that the azo compounds are synthesized generally by inducing a coupling reaction of a “diazonium compound” and a “coupler compound.” For the diazonium compound and the coupler, a various kind of compounds with difference structures are used depending on the intended application for azo dye or azo pigment. The types of the compounds selected for the diazonium compound and the coupler compound determine such properties of the obtained azo dye or azo pigment as an absorption wavelength and its intensity, water-solubility, light fastness, and resin dispersion.
Conventionally, there have been known the azo compounds used for these types of photoconductors, for example, benzidine bis-azo compounds disclosed in the JP-A No. 47-37543 and No. 52-55643, a stilbenzene bis-azo compound disclosed in the JP-A No. 52-8832, a diphenylhexatriene bis-azo compound disclosed in the JP-A No. 58-222152, diphenylbutadiene bis-azo compounds disclosed in the JP-A No. 58-222153. Further, the compounds, which have been known for the better properties, include the azo compound having a carbazole skeleton (disclosed in the JP-A No. 53-95033), the azo compound having a distilylbenzene skeleton (disclosed in the JP-A No. 53-133445), the azo compound having a triphenylamine skeleton (disclosed in the JP-A No. 53-132347), the azo compound having a dibenzothiophene skeleton (disclosed in the JP-A No. 54-21728), the azo compound having an oxadiazole skeleton (disclosed in the JP-A No. 54-12742), the azo compound having a fluorenone skeleton (disclosed in the JP-A No. 54-22834), the azo compound having a bis-stilbene skeleton (disclosed in the JP-A No. 54-17733), the azo compound having a distilyloxadiazole skeleton (disclosed in the JP-A No. 54-2129), the azo compound having a distilylcarbazole skeleton (disclosed in the JP-A No. 54-14967), and the like.
Further, there have been known the coupler compounds used for the above-mentioned compounds including a naphthol coupler (disclosed in the JP-A No. 47-37543), a benzocarbazole coupler (disclosed in the JP-A No. 58-122967), a naphthalimide coupler (disclosed in the JP-A No. 54-79632), a perinon coupler (disclosed in the JP-A No. 57-176055), an azulene coupler (disclosed in the JP-A No. 60-10256), an anthracene coupler (disclosed in the JP-A No. 61-257953), and the like.
Nevertheless, when used for a laminated photoconductor, the conventionally used azo compound, which is one form of photoconductors for electrophotography, generally do not have always sufficient sensitivity and durability to put into practical use. Hence, further improvement in their sensitivity and durability is desired to meet a various types of requirements essential for the electrophotographic process.
The inventors of the present invention have done a great deal of research to solve the above-mentioned subject and succeeded in obtaining such a finding that the Electrophotographic photoconductor containing a novel azo compound using a specific coupler mentioned later has sufficient sensitivity and durability for practical use.
On the other hand, from the viewpoints of the structure of and charging process of a photoconductive layer, most of practically applied photoconductors have laminated compositions, which are composed of a layer (CGL) having a charge generating function and a layer (CTL) having a charge transporting function, and are particularly used in the negatively-charging process. That is because: {circle around (1)} the laminated composition achieves mechanical strength and enables the photoconductors to retain sufficient mechanical strength during undergoing the process by disposing the CTL, for which film thickness can be designed, on its surface top layer, and {circle around (2)} The organic materials having enough charge mobility to use even in the high-speed copying process with no difficulty are, at present, almost limited to donor compounds exhibiting hole mobility. Therefore, the photoconductor has such a composition that the CTL formed by the donor compound is disposed on its surface side, which achieves negative electrostatic polarity. This type of function-separated composition, however, has given new problems.
The first one of these problems is derived from negatively charged photoconductors. A reliable charging method in the electrophotographic process is corona charging or contact charging, either of which is used for most copiers and printers. Nevertheless, as known well, negative charging is less stable than positive charging. Furthermore, negative corona charging involves generation of larger amounts of ozone and NOx, both of which are substances causing chemical damages, leading to problems such as environmental pollution and damages to photoconductors. In addition, in the contact charging method, which involves generation of far less amounts of ozone and NOx, a close approach to the photoconductors is needed, leading to considerable damages to photoconductors themselves.
The second one of the problems is derived from the laminated structures of the photoconductors. In manufacturing the photoconductors using organic materials, the solution coating method, which is more economical than the vacuum evaporation method, can be used. Although two coating operations are required to manufacture this laminated type of photoconductors, in general, three coating operations are required to dispose an interlayer on a photoconductive support (between the photoconductive support and the photoconductor) to ensure the electrostatic property of the photoconductor and this requirement for several times of coating operations increases the manufacturing cost of photoconductors. Moreover, the requirement for ensuring a better balance between sensitivity and durability, as well as another requirement for controlling the thickness of the GCL within the order of sub-microns to achieve better images are factors for further increasing the manufacturing cost.
Considering these problems, it can be understood that the photoconductors using organic materials have preferably a single-layer composition, which can be used in the positive charging process. Further, it can be understood that if the photoconductor can be used with no or a slight modification in the negative charging process, it is possible that the inexpensive photoconductors having an advantage of being considerably free to use in any environment will be newly manufactured.
Conventionally, there have been known the single-type of photoconductors including {circle around (1)} a charge mobile complex photoconductor made of polyvinylcarbazole and trinitrofluorenone (disclosed in the specification of U.S. Pat. No. 3,489,237), {circle around (2)} an eutectic complex made of thiapyrylium dye and polycarbonate (described in j. Appl. Phys. 49 5555 (1978)) and {circle around (3)} a photoconductor having perilene pigment and a hydrazole compound dispersed in a resin (disclosed in the JP-A No. 2-37354).
Among them, the photoconductors described in {circle around (1)} and {circle around (2)} have low sensitivity, as well as low electrostatic and mechanical durability, which gives a problem when it is repeatedly used. The photoconductor described in {circle around (3)} has a disadvantage of being unsuitable for the high-speed copying process because of its low sensitivity. Moreover, the system, in which components of the practically-used laminated photoconductor are simply dispersed, has a disadvantage of large variation in electrostatic properties when it is repeatedly used because of its low electrostatic potential and sensitivity, in particular, low light fastness and electrostatic and mechanical durability.
Thus, there is a subject that high sensitive and durability of organic materials for the single-layer photoconductors should be developed and in particular, for charge generating substances, higher light-fastness and durability are required than those used for the laminated photoconductors because of their charge generating point being on the surface top sides of the photoconductive layers unlike the laminated photoconductors.